Previous work from our laboratory has led to the conclusion that in the Ehrlich-Lettre ascites tumor cell both chloride and sulfate are transported by a common mechanism possessing two reactive membrane sites. This conclusion is based on: 1) the effect of sulfate on chloride transport and chloride on sulfate transport; and 2) the effect of H2DIDS (4,4'-diisothiocyano-1,2 diphenyl ethane, 2,2'-disulfonate) on the transport of both anions. Reversibly bound H2DIDS inhibits transport of sulfate to a greater extent than chloride. However, covalently bound H2DIDS inhibits transmembrane movement of sulfate but is without effect on chloride. This suggests that H2DIDS interacts irreversibly with that component of the anion transporter concerned with the regulation of sulfate permeability. We have recently synthesized tritium labeled-H2DIDS and have determined the relationship between 3H2DIDS binding and inhibition of sulfate transport. Two membrane proteins, 93K and 99K MW have been implicated in the transport process. In the coming year we plan to establish the Ehrlich cell in tissue culture and measure the turnover rate of the membrane proteins.